Flush head fasteners are used where surfaces must be smooth and free of protrusions, for example, the Aerodynamic surfaces of aircraft; where, in order to reduce drag, flush head fasteners are almost exclusively used. The fastener hole is countersunk at a total included angle of around 100 degrees for metals and 130 degrees for composites with the total underhead included angle of the fastener made approximately equal thereto. In order to achieve a smooth aerodynamic contour, the countersink overall diameter is usually made slightly larger than the diameter of the fastener head such that upon installation, the top surface of the fastener head will e below the aerodynamic surface. This is also necessary because the fastener at the junction of the upper and underhead surface of the head normally includes a small cylindrical land to eliminate any sharp edges that would otherwise exist at the junction thereof. What is usually left upon installation is a small countersunk ring about the fastener head. Of course, if the fastener is being installed on a curved surface, the countersunk hole will be elliptical in shape and thus the gap around the fastener head becomes uneven. These sort of gaps are commonly called "eyebrows". Another surface discontinuity is the typical internal wrenching recess in the fastener head, i.e., phillips, slot, tri-wing, allen, etc. Thus, if it is necessary to have an absolutely smooth aerodynamic surface or if no electrical discontinuities can be tolerated, the recess and the exposed portion of the countersunk hole "eyebrows" must be covered.
Typically, putties are used; conductive ones if electrical continuity is required. Another method is to use covers bonded to the fastener head or retained by means of a protrusion on the cap which is jammed into the internal wrenching recess in the fastener head. Neither of these methods provide any guarantee of a smooth external contour and, of course gaps and discontinuities, while reduced in size, will always be present.
The internal wrenching recess can be eliminated if the fastener can be torqued from the shank end. For example, the internal wrenching recess can be provided at the threaded end of the fastener. However, in many applications, access to the opposite end of the fastener is restricted so this solution is not often viable. Another approach is to use detachable (frangible) external torquing surfaces attached to the head of the fastener designed to break off after the proper torque level is reached. Such a fastener has several disadvantages: there is no way of removing it except by means of an easy out (requiring drilling into the fastener head itself) and a rough external head surface remains. Thus, such fasteners are not used on external aerodynamic surfaces.
Another problem occurs when the aerodynamic surface is made of composite materials, such as graphite fiber reinforced epoxy resins. When used in the wings of aircraft which contain fuel, a significant safety problem can exist; for while the structure of the wing may be made of composite materials, the fasteners used are typically metal in order to obtain sufficient structural strength. The difference in electrical conductivity between the composite material and the fastener causes lightning to become attached to the metallic fastener head and the fastener can conduct current into the interior of the wing and cause internal arcing inside the fuel tank. If fuel vapors are present in the tank, such arcing can cause an explosion. Therefore, it is necessary to provide some sort of protection for external metal fasteners used in composite structures to prevent such lightning strikes attaching thereto.
Typically, a coat of paint is applied over the aerodynamic surfaces. However, the gap between the fastener head and its countersink hole can cause the paint to thin and crack around the outer edge of the fastener. This thinning and cracking of the paint in turn causes a difference in electrical conductivity, which causes electrical streamers to form around the edges of the fastener heads. These streamers create an electrical field and increase the chance of a lightning strike hitting a fastener head. To eliminate this possibility various devices to cover the fastener head have been proposed. For example, U.S. Pat. No. 4,630,168 "Lightning Protection Fastener" by J. Hunt, discloses a fastener having a metallic head and shaft and a dielectric cap covering a top portion of the head. When the head and cap are installed into the countersunk hole in the skin, the cap being resilient tends to fill the gap between the top portion of the head and the inner side walls of the countersink. The cap forms with the skin an aerodynamic surface which is sufficiently continuous to receive and maintain a crack free coat of paint having a uniform thickness.
The problem here is that the cap encompasses a portion of the underhead conical surface and when installed is wedged between the fastener head and the countersunk hole. This presents a problem in heavily loaded structures in that the resilient material between the fastener head and countersink will tend to extrude under cyclic loading and the fastener may eventually become loose fitting therein. Additionally, since the cap is installed with a given thickness and tolerance it may not always provide an absolutely smooth external contour. Another approach to providing protection for metal fasteners in composite structures is found in U.S. Pat. No. 4,681,497 "Encapsulated Fastener" by I. Berecz. Here the head and shank are encapsulated in composite material and thus, it is subject to the same deficiencies as the preceding example. Another approach can be found in U.S. Pat. No. 4,502,092 "Integral Lightning Protection System for Composite Aircraft Skins" by E.T. Bannink, Jr. et al. Here a plastic strip is placed over the outer surface and a potting compound is used to fill the space over the fastener. In U.S. Pat. 4,628,402 "Lightning Protection of Fasteners in Composite Material" by J.H. Covey, one embodiment uses a rubber plug over the fastener to fill the space above the fastener caused by the layers of dielectric material. None of the above fastener systems designed for use with composite materials are suitable for use with metal surfaces. Thus, there is no available fastener design that can provide a smooth continuous aerodynamic surface in both composite and metal structures and which does not require a paint be applied over the surface, etc.
Thus, it is a primary object of the subject invention to provide a flush head fastener that is capable of producing an aerodynamically smooth external surface.
It is another primary object of the subject invention to provide a flush head fastener that is capable of providing an aerodynamically smooth external surface when the surface is curved.
It is another object of the subject invention to provide a flush head fastener that produces no electrical discontinuities on the surface after installation.
It is a further object of the subject invention to provide a flush head fastener that can be used with both metal and composite materials.